The present invention relates to vertical cavity surface emitting lasers (xe2x80x9cVCSELsxe2x80x9d). More particularly, the present invention relates to VCSELs having improved electrical conduction properties and their method of manufacture and operation.
Vertical cavity surface emitting lasers (xe2x80x9cVCSELsxe2x80x9d) are ideal sources for two-dimensional array applications such as optical scanners, displays, computer interconnects, signal processing and optical data storage. VCSELs also find applications as light sources in computing equipment, laser printers, consumer electronics systems, active optical components and communications applications. Compared with other semiconductor laser technologies, such as edge emitter lasers, VCSELs are increasingly preferred because they emit light having a circular anastigmatic beam and relatively limited angular divergence. On the other hand, the laser beam from an edge-emitter laser tends to be asymmetric. In addition, VCSELs are preferred to edge-emitter lasers because VCSEL light output emerges from the top of the structure in a beam normal to the semiconductor substrate, e.g., a wafer. This configuration favors cost-effective wafer-scale testing and the production of laser array devices, which is an increasingly popular device format in communications systems.
Conventional VCSELs typically produce a Gaussian output optical intensity distribution when the output power is limited, typically to less than 1 milliwatt (mW). When such a VCSEL is operated at a higher power level, the device exhibits multi-mode operation, and the optical output intensity pattern degrades to a multi-mode, doughnut-shaped distribution. Lenses are typically used to concentrate the output of a VCSEL to couple the light into an optical fiber or a waveguide, but the efficiency of these lenses is limited when the VCSEL operates in a multi-mode range.
In many of today""s communications applications, VCSELs are required to operate in single mode. Optical and electrical power efficiency in single mode operation reduces the operating cost as well as heat dissipation complications. Heat dissipation is increasingly important as advances in the technology of communications systems increase demands for higher density integrated VCSEL arrays. Hence, VCSEL designers strive to produce lasers that emphasize single mode operation while minimizing higher mode output.
FIG. 1 shows a cross-sectional side view of a conventional VCSEL. The conventional VCSEL structure 10 comprises a semiconductor substrate 30, a vertical laser cavity 40 built on top of the semiconductor substrate 30, and two metal contacts 20, 25. The metal contacts 20, 25, generally opaque in nature, sandwich the entire substrate and the vertical laser cavity structure, one being on top of the vertical laser cavity 40 while the other being below the semiconductor substrate 30. The metal contact 25 is annular in shape and resembles a circular ring or an elliptical ring when viewed from top.
The vertical laser cavity 40 further comprises an n-distributed Bragg reflector (DBR) section 42, a p-DBR section 46, an active lasing section 44 sandwiched between the two DBR sections 42, 46, and a non-conductive implant material 48 surrounding a middle portion 47 of the p-DBR section 46. The non-conductive implant material 48 forms a conductive boundary wall 50 for the VCSEL structure 10. A cylinder that includes two parts defines a barrel region 52. The first part comprises the middle portion 47. The second part comprises a portion of the active lasing section 44 that is coaxially aligned with and has substantially the same cross-sectional configuration as the middle portion 47. This barrel region 52 has a diameter 60.
In operation, light is emitted from the active lasing section 44. A portion of the light energy tunnels through, first, the barrel region 52 and then through an opening/aperture 65 defined by the opaque upper metal contact 25. The diameter 60 of the barrel region is constrained by the need to provide good current conduction between the upper metal contact 25 and the barrel wall. This is true because the non-conductive boundary wall of the VCSEL structure 10 does not convey current between the metal contacts 20, 25. This constraint is usually addressed by creating a metal contact overhang 70 as an extension of metal contact 25 at the top of the VCSEL structure 10. As a result, the metal contact 25 on the top of the VCSEL structure 10 overhangs the barrel region. The opening/hole 65 defined by the metal contact overhang 70 sets the aperture diameter for the conventional VCSEL structure 10. Therefore, the device aperture of the conventional VCSEL structure 10 is necessarily smaller than the diameter 60 of the barrel region 52.
The metal contact overhang 70 obstructs light emitted from the barrel region 52, and thus limits output energy. The metal contact overhang 70 also causes inefficient current delivery to the barrel region 52. The current flow in the VCSEL structure 10 will hug the walls of the laser and induce relatively little lasing activity along the length of the VCSEL structure 10 to promote single mode operation.
Another challenge of VCSEL architecture posed by the upper contact overhang 70 is back scattering, which compounds the aforementioned inefficiency of the metal contact overhang 70. The upper metal contact overhang 70 blocks a portion of outbound light and reflects the light back into the laser barrel. This reflected or back-scattered light interferes constructively with outbound light in the barrel region and establishes a standing wave pattern that contributes significantly to inducing undesirable multi-mode output. The back scattering affect results in the multi-mode, donut-shaped output intensity pattern that is characteristic of a standard VCSEL.
Several VCSEL manufacturers have sought to reduce multi-mode output by filtering or blocking higher mode operation. Spatial absorptive filters and/or phase shift filters are integrated into this type of VCSEL. One example of such VCSEL is described in U.S. Pat. No. 6,144,682. FIG. 2 shows a cross-sectional side view of the VCSEL described in U.S. Pat. No. 6,144,682. This VCSEL exhibits an effort to reduce modal reflectivity for the higher order transverse modes of the emitted light from the active lasing section 144 of the VCSEL. This is accomplished by using thicker and thinner spatial absorptive and phase shift filter semiconductor layers 182, 180 between the p-DBR section 146 and upper metal contact 125. The VCSEL further utilizes a translucent, conductive Indium Tin Oxide layer 190 to assist electric current to flow through the p-DBR section 146 and into the active lasing section 144. However, the upper metal contact 125 is still required to extend into the barrel region of the VCSEL, as shown by region 170, to provide an even current flow from the upper metal contact 125 into the barrel region. Similar to FIG. 1, the diameter of the opening 165 defined by the upper metal contact 125 is smaller than the diameter 160 of the barrel region as confined by the non-conductive implant material 148. As a result, output light is obstructed, and undesirable back scattering occurs. Although filtering structure could somewhat compensate for the undesirable multi-mode output, the VCSEL as shown in FIG. 2 does not favor single mode operation.
Aspects of the present invention relate to apparatus and methods for improving the current conduction structure of a VCSEL. A layer of heavily doped semiconductor material is disposed over the top surface of a distributed brag reflector (DBR) on top of the VCSEL structure. This material provides current conduction and current spreading across and into the aperture of the laser barrel. The current flow in the center of a laser barrel region is increased relative to the barrel walls, favoring single mode light production. The heavily doped semiconductor layer also eliminates conductive electrical contact overhang that obstructs, increasing output energy. Because the conductive electrical contact overhang is eliminated, scattered light is not reflected back into the barrel region of the laser, and the multi-mode light induction effect is minimized. This feature leads to improved emission efficiency and smaller diameter laser structure that favors single mode light operation at relatively high power.